1. Field of the Invention
The invention relates to a communications network and, in particular, to a method and system that transmits messages between devices of a mobile radiotelephone network.
2. Description of the Related Art
A digital, cellularly constructed mobile radiotelephone network according to the Global System for Mobile Communication Standard (GSM) is disclosed in the system documentation entitled "D900--Mobile Communication System. System Description SYD" 1992. Essential components of the radiotelephone network for transmitting speech and data include a base station system, a switching subsystem and an operating and maintenance subsystem. The base station system having a radio part and a wired part interfaces with the mobile stations of radiotelephone subscribers and the switching subsystem. The switching subsystem controls the switching functions in the mobile radiotelephone network. This includes setting up the connection between within the mobile radiotelephone network and other telecommunications networks on the basis of calls originating at or received by the radiotelephone subscribers.
The GSM specification 09.02, final draft, entitled "European Digital Cellular Telecommunications System (Phase 2)" November 1994, specifies a standardized transmission protocol for the international mobile radiotelephone network (Mobile Application Part--MAP), in order to standardize the handling of the special characteristics resulting from the mobility of the radiotelephone subscribers, or the corresponding mobile stations, during calling or data exchange across state borders. Messages are transmitted between the devices of the mobile radiotelephone network according to the standardized transmission protocol (MAP).
In the GSM mobile radiotelephone network, subscriber call numbers are assigned to the radiotelephone subscribers. The assigned call numbers are part of the subscriber data that is stored in a central subscriber database. A combination of digits positioned at the beginning of the subscriber call number, allows a call connection to always be made to the same determined central subscriber database in the mobile radiotelephone network. A location update of the radiotelephone subscriber, provides for a branching to take place to transmit the data to the correct central subscriber database based on the first digits of an international radiotelephone subscriber identification (IMSI). The IMSI is entered during the manufacturing of the Subscriber Identity Module (SIM) card and is allocated to the radiotelephone subscriber before the radiotelephone subscriber is set up in the network. The subscriber call number is not assigned until the radiotelephone subscriber is cleared for set up in the network.
It is not possible to freely allocate a subscriber call number for a radiotelephone subscriber that has a combination of digits to switch the call through to a subscriber database that is different than the determined central subscriber database without an increase in cost. In addition to the cost of having the desired subscriber call number set up, a cost increase is necessary in order to set up an unconditional call forwarding number that corresponds to the subscriber call number containing the combination of digits identifying the predetermined subscriber database for the radiotelephone subscriber. The last-named subscriber call number is not outwardly visible, but is used by the mobile radiotelephone network operator for an assignment to a different radiotelephone subscriber. As a result two subscriber call numbers are used to identify the radiotelephone subscriber from the possible plurality of subscriber call numbers available to the network operator. This additional performance feature of call forwarding is incorporated prior to the radiotelephone subscriber's request for the feature. As a result problems arise with respect to displaying the subscriber call number as well as determining the fees to be charged for the respective call.